1. Field of the Invention
The invention relates generally to seismic surveying and, more particularly, to a method for positioning seismic sources and seismic receivers in performing seismic surveys.
2. Description of the Related Art
Seismic surveying is used for determining the structure of subterranean strata. Seismic surveying typically uses a seismic energy source, such as explosive charges or mechanical vibrators, and seismic receivers, such as geophones or accelerometers. The seismic energy source generates acoustic waves which propagate through the subterranean strata and reflect from acoustic impedance differences generally at the interfaces between strata. The reflected waves are detected by the seismic receivers, which generate representative electrical signals. The resulting signals are transmitted by electrical, optical, or radio telemetry to a location where the signals are recorded for later processing and interpretation. The measured travel times of the reflected waves from the source to the receiver locations and the characteristics of the received energy, such as amplitude, provide information concerning the subterranean strata. Seismic surveys are interpreted to determine the most suitable locations for drilling wells for production of hydrocarbons.
The seismic receivers detect noise from many sources known in the art, and detect multiple reflections, as well as the primary reflected waves which are of interest in determining the subsurface structures. The noise and multiple reflections obscure the desired signal and complicate the process of seismic data analysis. A common technique for enhancing the signal-to-noise and primary-to-multiple ratios is the use of multiple"fold" data. This technique activates the seismic source at a plurality of locations for detection by multiple seismic receivers. The seismic signals received over time are"gathered" by identifying those seismic signals or"traces" corresponding to the same subsurface reflection point, such as a common depth point (CDP) or a common midpoint (CMP). The traces in each gather are "stacked". Stacking is the process of summing together the traces so that the coherent primary signal is enhanced by in-phase addition while source-generated and ambient noise are attenuated by destructive interference. The number of traces in each common point gather is termed the fold or multiplicity.
Two-dimensional (2-D) seismic surveys typically utilize a simple linear recording geometry. A receiver "group" of one or more receivers is positioned at each receiver station, or location, and the receiver locations are arranged in a single line. The receiver locations are typically equally spaced along the receiver line, giving a constant group interval, or spacing, between receiver locations. The source stations or locations are generally collinear or parallel to the receiver line. Multiple fold data is obtained by moving the source location relative to the receiver line so as to maintain a common depth point for multiple pairs of source and receiver locations. The source locations are typically equally spaced, giving a constant source interval or spacing between source locations.
Three-dimensional (3-D) seismic surveys utilize more complex recording geometries. 3-D recording geometries known in the art typically use multiple parallel receiver lines of seismic receivers, typically with the receiver locations equally spaced along the receiver lines and the receiver lines equally spaced from each other. Source locations are typically positioned along source lines and typically are evenly spaced. The source lines are typically orthogonal to the receiver lines, but may also be parallel to or at a diagonal angle, typically 45 degrees, to the receiver lines. In 3-D surveys, gathers are constructed by taking all seismic traces from an area, referred to as a "bin", around each common midpoint and assigning the traces to that common midpoint. The areal dimensions of the bin are generally half the group interval by half the source interval. The size of the source interval is independent of the size of the group interval, allowing the use of rectangular bins rather than square bins. Seismic recording methods using these geometries are generally termed "swath" methods. As data are recorded along one swath, one or more of the receiver lines are picked up and replaced on the other side of the recording spread to be used in the next swath, a process termed rolling, rolling along, or rolling over. A uniform fold, in which each rollover develops the same positive integer value for multiplicity, is termed an even fold. Maintaining an even fold constrains the number of receiver lines recorded, the number of receiver lines which are rolled over each time, and the location of sources relative to the receiver spread. Increasing the fold requires increasing the number of receiver lines or decreasing the source line interval, thus increasing the number of source locations.
An alternative 3-D recording geometry, called a button patch or patch system developed by Atlantic Richfield Company (ARCO), is described in Crews, G. A. et al., An Economical High-Resolution 3-D Seismic Survey Technique, Extended Abstracts, 61st Annual International Meeting, Society of Exploration Geophysicists, 90, 863-866 (1991). Seismic receivers are positioned in multiple sets of rectangular patches of receiver groups. The patches are called buttons and are interspaced in checkerboard fashion with rectangular empty spaces called button-holes. The sources are positioned at various locations both within and outside the button patch. These and other recording geometries are described in Stone, D. C., Designing Seismic Surveys in Two and Three Dimensions, Society of Exploration Geophysicists, Tulsa (1994).